Heme oxygenase, the rate-limiting enzyme in the mammalian heme degradative pathway, catalyzes the oxidation of heme into biliverdin, which is readily converted to bilirubin by biliverdin reductase. Bilirubin, due to its inherent antioxidant properties, can protect cells against oxidative damage. Oxidative stress has been shown to markedly induce heme oxygenase activity in several tissues. This response is now thought to be a cellular defense mechanism against oxidative stress. Heme oxygenase consists of two isozymes- an inducible heme oxygenase-1 (HO-1) and a noninducible heme oxygenase-2 (HO-2). Since the retina is often subject to oxidative stress, we analyzed rat retina for the possible presence of HO-1 and HO-2. The expression of HO-1 and HO-2 mRNA in retina was analyzed using a cDNA-PCR technique. The mRNA fraction prepared from rat retina was reverse transcribed and used as template for PCR using oligonucleotide primers specific for rat HO-1 or rat HO-2. Both HO-1 mRNA and HO-2 mRNAs were found to be present in retina by this technique. HO-1 protein was also detected in the rat retina when total proteins from this tissue was subjected to gel electrophoresis followed by Western blot analysis using an anti HO-1 antibody preparation. Studies were also conducted using a human retinoblastoma Y-79 cell line as an in vitro model system. Western blot analysis showed the presence of HO-1 in these cells. HO-1 content in Y-79 cell's was markedly increased following treatment with sodium arsenite, menadione or heme. The mRNA for HO-1 was detected in Y-79 cells by Northern blot analysis using a human HO-1 probe. The HO-1 mRNA content in the cells was elevated upon treatment with sodium arsenite or menadione. The expression of HO-1 mRNA in Y-79 cells was also indicated by a cDNA-PCR technique. The ability of Y-79 cells to degrade heme into bilirubin was increased following the sodium arsenite treatment as shown by HPLC. Thus it appears that HO-1 and HO-2 are expressed in rat retina. Also, HO-1 is expressed in Y-79 cells, and the expression is enhanced in response to oxidative stress.